How many different calendar systems are used in the world?

There are around 40 different calendar systems that are used around the world today. Some of the most common calendar systems include the Gregorian calendar, the Chinese calendar, the Islamic calendar, and the Hebrew calendar. The Gregorian calendar is the most widely used calendar in the world and is the civil calendar of most countries. The Chinese calendar is a lunisolar calendar that has been used in China for thousands of years. The Islamic calendar is a lunar calendar that is used by Muslims around the world. The Hebrew calendar is a lunisolar calendar that is used by Jews around the world.